The subterranean life of the cicada nymph is an unusual phase in the insect world. This stage can last for years, with the nymph spending its existence burrowed deep underground before emerging as an adult. Unlike many insects that feed on decaying matter or solid roots, the cicada nymph has a highly specialized diet and feeding method. This unique lifestyle allows them to develop slowly over many seasons, acquiring the necessary energy for their final transformation.
The Exclusive Diet of Cicada Nymphs
Cicada nymphs feed exclusively on xylem sap, a plant fluid they extract from the roots of trees and woody plants. Xylem is the vascular tissue that transports water and dissolved trace minerals from the roots up to the rest of the plant. This fluid is extremely dilute, consisting primarily of water with small concentrations of organic molecules like amino acids and carbohydrates.
This diet contrasts sharply with phloem sap, the other main plant fluid, which is rich in sugars produced during photosynthesis. Because xylem sap is nutrient-poor, cicada nymphs must consume vast quantities continuously to acquire the necessary proteins and energy. To overcome these nutritional deficiencies, the nymphs rely on symbiotic bacteria housed within specialized cells. These bacteria synthesize the essential amino acids and vitamins missing from the xylem sap, allowing the insect to thrive on this restrictive food source.
Specialized Feeding Underground
Nymphs find their food source by burrowing through the soil until they locate the root systems of suitable host trees and shrubs. Once a fine root is located, the nymph becomes relatively sedentary, attaching itself to the root to begin feeding. The feeding mechanism involves a complex, tube-like mouthpart called a rostrum, which is characteristic of true bugs.
The rostrum houses needle-like stylets, which the nymph uses to pierce the external root layers and tap directly into the xylem vessels. Unlike other insects that feed on phloem sap, the xylem fluid they consume is under negative pressure as it is pulled up the plant. The cicada nymph must exert significant suction pressure to draw the watery sap up through its mouthparts. This suction is achieved through large, powerful pumping muscles located in the insect’s head.
The Long-Term Effects on Host Trees
The duration of the nymph stage varies greatly depending on the species. Annual cicadas spend two to five years underground, while the famous periodical cicadas can spend 13 or 17 years as subterranean feeders. Despite feeding on the root system for such extended periods, the localized consumption of xylem sap rarely causes significant or lethal damage to healthy, mature host trees. The feeding is considered a form of chronic, minor herbivory that the plant can generally tolerate.
However, the feeding can weaken the vigor of smaller, younger trees or those already stressed by other factors. A heavy concentration of nymphs can disrupt the tree’s ability to absorb water and nutrients, potentially leading to stunted growth. This root-feeding damage is distinct from the more noticeable damage caused by adult female cicadas, which make slits in small branches to lay their eggs, sometimes causing the tips to die and break off.